Touch the Past
M useum s Education Service
Why do Boats Float?
Key Stages: l, 2 & 3
National Curriculum Links: Sc1 1, 2a,b,c,e,f,g,h,i,j,k,l Sc3 1a,b,c,e Sc4 2a,b
Main Subject Focus: Science
Overview:
Before it sank the Titanic was an awesome sight, it was about 269 metres long
(almost 3 football fields long!) and weighed 47,072 tonnes. But how could such an
enormous craft stay afloat in the first place? This intriguing question will be answered in this activity. Students will learn about the concepts of mass, volume
and density, and apply these to several ordinary real-Iife situations.
Learning Outcomes:
Children will:
• Know that some materials are able to float whilst others will sink,
• Know why some float and others sink
• Students will understand that objects which are less dense than water
can float
• Students will observe how air helps dense materials to float
• Be able to carry out a scientific investigation, predict and test their predictions
• Be able to record their results accurately
Resources required:
• Clear Storage Box
• Water
• Large selection of materials for children to test
• Weighing Scales
• Towels
• Worksheets & pencils
Activity Structure:
1. Ask the children what materials they think boats past and present have
been made from. Ask them why huge metal ships don't just sink.
2. Show the children the selection of objects. Discuss with the children what
the objects are made from.
3.
4.
5.
6.
Ask the children to predict which objects will sink and which will float. The
students should record their predictions on Worksheet 1.
Now ask the children to test their predictions by placing the objects oneby-one into the water. They should record their results on the worksheet.
Once all the objects have been tested, discuss the results on the worksheet and ask if there were any surprising results e.g. nail=metal, ships
are metal - how do they float?
Ask the children to experiment with the ball of plasticine - can they make it
float? If they can make it float ask them why they think it floats.
Extension for KS 2 & 3
Students will investigate why ships such as the Titanic float despite being made
of materials like steel, which is more dense than water. Read the definitions of
mass, weight, volume and density, from the pupil's information sheet, to the students before you begin. Students may record their results and predictions on
Worksheet 2.
1. Ask the students to weigh the metal spoon and the empty tin can.
Point out to the children that the objects have about the same mass.
2. Now ask the students to put both items into the water. The tin can will
float and the spoon will not. Ask the students whether the spoon is
more or less dense than water. They should discover that the spoon
must be more dense that water because it does not float.
3. Ask the students why the tin can floats, even though its mass is the
same as the spoons. Students should see that the can must have a
lower density that the spoon. Ask the students if the volume of the can
is less or more than the spoon. The answer should be that the can's
volume is higher than the spoon's because it takes up more space. Next
ask the students to think about what the tin can has that the spoon
does not. The answer should be that the tin can has air.
4. Finally ask students to predict whether air would be more or less
dense than water. Air is less dense than water because air does not sink
in water!
5. If it has not sunk already 1 fill the can with water and ask the students what will happen when you let go. The answer should be that it
will sink! Ask the students to explain why it no longer floats. The can
no longer contains air, which is less dense than water. Instead it just contains metal which is more dense than water, (and water which is, of
course, the same density as water!) and so sinks.
6. Finally ask the students to use this experiment to explain why the Titanic sank when its hull was breached and water poured into the
ship. Students should be able to understand that a ship floats because its
total density - composed of the density of all its materials, such as wood,
steel, and air - is less than the density of water. When the air on the ship
is replaced by water, the density of the ship becomes greater than water.